Optical module

ABSTRACT

An optical module includes: a wiring substrate that has a wiring pattern including a connecting portion and is arranged on an optical subassembly so as to be electrically connected thereto; and a flexible insulating layer formed between the optical subassembly and the wiring substrate. The optical subassembly includes: a conductive stem that has a surface opposed to the wiring substrate, the conductive stem being shaped so that the surface has a through hole opened therein and being connected to a reference potential; and a signal lead for transmitting a signal, the signal lead passing through the through hole while being electrically insulated from the conductive stem. The signal lead passes through the flexible insulating layer to be joined to the connecting portion. The flexible insulating layer is in contact with the connecting portion, the signal lead, and the surface of the conductive stem.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese applicationJP2015-118924 filed on Jun. 12, 2015, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical module.

2. Description of the Related Art

A CAN-type optical module used in optical communication generallyincludes an electrically-grounded stem and a lead terminal that passesthrough the stem and is insulated from the stem. The stem and a capmounted to the stem form a housing for accommodating an opticalsemiconductor device. The lead terminal and the stem form a coaxialline. One end portion of the lead terminal is connected to the opticalsemiconductor device. Another end portion of the lead terminal isconnected to a drive device configured to output a modulated electricalsignal, via a wiring substrate, e.g., an FPC having a signal line and aground formed along the signal line.

Increases in communication speed are leading to an increase in thefrequency of the modulated electrical signal output from the drivedevice. Due to the increase in the frequency of the electrical signal,reflection of the electrical signal tends to occur in a transmissionline at locations at which there are mismatches in the characteristicimpedance. Characteristic impedance mismatches tend to occur between,for example, the coaxial line, which is formed by the lead terminal andthe stem, and the wiring substrate, e.g., an FPC. At locations in thetransmission line at which there are mismatches in the characteristicimpedance, reflection waves of the electrical signals are generated tointerfere with original modulated electrical signals, resulting inlowered waveform quality of optical signals. In order to suppress to aminimum the effects of characteristic impedance mismatches on theoptical waveform at a connecting portion between the lead terminal andthe FPC at which characteristic impedance mismatches tend to occur, inJapanese Patent Application Laid-open No. 2009-302438, there areproposed structures for suppressing characteristic impedance mismatches.

Specifically, in Japanese Patent Application Laid-open No. 2009-302438,a flexible printed board, which is vertically mounted on a CAN-typepackage in the related art, is horizontally connected to the CAN-typepackage, thereby suppressing impedance mismatches.

In recent years, optical modules have been strongly demanded to achievenot only a lowered cost but also an increased speed. If high-speedoptical signals in the 25 Gbit/s class can be transmitted withinexpensive CAN-type optical modules, both the demands for a loweredcost and an increased communication speed can be satisfied. However, ifa unique flexible printed board connection method, e.g., the oneproposed in Japanese Patent Application Laid-open No. 2009-302438, isemployed for suppressing impedance mismatches, soldering is complicated,thereby influencing processing costs and yields of optical modules. As aresult, the optical modules may not be stably manufactured and supplied.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a mode of aninexpensive optical module that is capable of transmitting or receivinghigh-speed optical signals, is excellent in high-frequencycharacteristics, and does not increase a cost.

(1) An optical module according to one embodiment of the presentinvention includes: an optical subassembly configured to convert atleast one of an optical signal or an electrical signal into another oneof the optical signal and the electrical signal; a wiring substrate thathas a wiring pattern including a connecting portion and is arranged onthe optical subassembly so as to be electrically connected thereto; anda flexible insulating layer formed between the optical subassembly andthe wiring substrate, the optical subassembly including: a conductivestem that has a surface opposed to the wiring substrate, the conductivestem being shaped so that the surface has a through hole opened thereinand being connected to a reference potential; and a signal lead fortransmitting a signal, the signal lead passing through the through holewhile being electrically insulated from the conductive stem, the signallead passing through the flexible insulating layer to be joined to theconnecting portion, the flexible insulating layer being in contact withthe connecting portion, the signal lead, and the surface of theconductive stem. According to the present invention, a space between theconnecting portion and the signal lead, a space between the connectingportion and the surface of the stem, and a space between the signal leadand the surface of the stem are filled with the flexible insulatinglayer, and hence neither a space with only air nor a vacuumed space isformed therein. Thus, parasitic inductance of the signal lead can bereduced.

(2) In the optical module according to Item (1), the wiring substratemay include an insulating substrate having a thickness corresponding toa distance between a first surface and a second surface of theinsulating substrate, the first surface maybe opposed to the surface ofthe conductive stem, and at least part of the connecting portion may beformed on the first surface.

(3) In the optical module according to Item (2), the connecting portionmay be formed on the first surface and the second surface so as to passthrough the insulating substrate, to thereby form a through-hole via forelectrical connection, and the signal lead may be inserted into thethrough-hole via.

(4) In the optical module according to Item (2) or (3), the wiringpattern may include signal wiring connected to the connecting portion,for transmitting the signal, and a plane layer for connection with thereference potential.

(5) In the optical module according to Item (4), the signal wiring maybe formed on the second surface, and the plane layer may be formed onthe first surface.

(6) In the optical module according to Item (4), the signal wiring maybe formed on the first surface, and the plane layer may be formed on thesecond surface.

(7) In the optical module according to any one of Items (2) to (6), thewiring substrate may include a protective film for covering the wiringpattern, and the wiring pattern may be formed on the first surface andthe second surface.

(8) In the optical module according to Item (7), the wiring patternformed on the first surface may overlap the conductive stem.

(9) In the optical module according to Item (7), the wiring patternformed on the first surface may overlap a region other than theconductive stem.

(10) In the optical module according to any one of Items (7) to (9), theprotective film formed on the first surface may overlap the conductivestem. (11) In the optical module according to any one of Items (7) to(9), the protective film formed on the first surface may overlap aregion other than the conductive stem.

(12) In the optical module according to any one of Items (1) to (11),the optical subassembly may further include a reference leadelectrically connected to the conductive stem in order to electricallyconnect the conductive stem to the reference potential.

(13) In the optical module according to any one of Items (1) to (11),the wiring pattern may be electrically connected to the conductive stemon the first surface, and the conductive stem may be electricallyconnected to the wiring pattern on a side surface of the conductive stemwhile avoiding the surface.

(14) In the optical module according to any one of Items (1) to (13),the through hole of the conductive stem and the signal lead may beelectrically insulated from each other with an insulating material, theinsulating material may be filled so that a projecting portion is formedthat protrudes from the surface of the conductive stem, and the flexibleinsulating layer may have a recessed portion in contact with theprojecting portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view for schematically illustrating an opticalmodule according to a first embodiment of the present invention.

FIG. 2 is a top view of the optical module according to the firstembodiment of the present invention when viewed from a wiring substrateside.

FIG. 3 is a sectional view of the optical module illustrated in FIG. 1taken along the line of FIG. 2.

FIG. 4 is a sectional view for illustrating the details of an opticalmodule according to a second embodiment of the present invention.

FIG. 5 is a sectional view for illustrating the details of an opticalmodule according to a third embodiment of the present invention.

FIG. 6 is a sectional view for illustrating the details of an opticalmodule according to a fourth embodiment of the present invention.

FIG. 7 is a sectional view for illustrating the details of an opticalmodule according to a fifth embodiment of the present invention.

FIG. 8 is a graph for showing a result of comparison between reflectioncharacteristics of the related art and the third embodiment by a highfrequency 3D electromagnetic field simulator (High Frequency StructureSimulator; HFSS).

FIG. 9 is a graph for showing a result of comparison betweentransmission characteristics of the related art and the third embodimentby the high frequency 3D electromagnetic field simulator (High FrequencyStructure Simulator; HFSS).

DETAILED DESCRIPTION OF THE INVENTION

Referring to the accompanying drawings, embodiments of the presentinvention are described below. In the drawings, the same or similarcomponents are denoted by the same reference symbols, and repetitivedescription thereof is omitted.

First Embodiment

FIG. 1 is a sectional view for schematically illustrating an opticalmodule according to a first embodiment of the present invention. Theoptical module includes an optical subassembly 10 configured to convertat least one of an optical signal or an electrical signal into theother. Examples of the optical subassembly 10 include an opticaltransmitter module (transmitter optical subassembly; TOSA) that includesa light emitting element, e.g., a semiconductor laser 12, and isconfigured to convert an electrical signal into an optical signal andtransmit the optical signal, an optical receiver module (receiveroptical subassembly; ROSA) that includes a light receiving elementrepresented by a photodiode and is configured to convert a receivedoptical signal into an electrical signal, and a bidirectional opticalsubassembly (BOSA) having both the functions of those opticalsubassemblies.

The invention of the subject application can be applied to any of theabove-mentioned optical subassemblies, and an optical transmitter moduleis described below as an example.

The optical subassembly 10 includes a conductive stem 14 formed, forexample, of metal. The stem 14 is connected to a reference potential(for example, a ground). A seat 16 is fixed to the stem 14. A heatdissipation substrate 18 is mounted on the seat 16 with the use of asolder material, a conductive adhesive, or others. The heat dissipationsubstrate 18 has high heat conductivity, and is formed of an insulatingmaterial having a thermal expansion coefficient close to that of thesemiconductor laser 12 (for example, aluminum nitride). A metal film(not shown) is formed on a surface of the heat dissipation substrate 18.The semiconductor laser 12 is supported by the seat 16 via the heatdissipation substrate 18.

A cap 20 is joined to the stem 14 so as to cover the semiconductor laser12. The stem 14 and the cap 20 form a housing 22 for accommodating thesemiconductor laser 12. The housing 22 is airtight. The cap 20 has anopening formed therein, and a lens 24 configured to collect light fromthe semiconductor laser 12 is arranged in this opening.

The optical subassembly 10 includes a signal lead 26 for transmittingsignals. The signal lead 26 is a terminal for transmitting modulatedelectrical signals to the semiconductor laser 12. One end portion of thesignal lead 26 is protruded from the stem 14 into the housing 22. Thisprotruded portion is connected, via a wire 28, solder, or others, to themetal film (not shown) formed on the surface of the heat dissipationsubstrate 18. The semiconductor laser 12 and the metal film (not shown)formed on the surface of the heat dissipation substrate 18 are alsoconnected to each other via the wire 28. Electronic components otherthan the semiconductor laser 12 (that are not shown) may also beprovided, e.g., a lead for supplying power that is connected to aphotodiode configured to monitor an output of the semiconductor laser 12or other components.

FIG. 2 is a top view of the optical module according to the firstembodiment of the present invention when viewed from the wiringsubstrate side. A plane layer 46, which is described later, cannot beactually seen because the plane layer 46 is formed on the rear side(stem 14 side) of an insulating substrate 38, which is described later.However, the plane layer 46 is illustrated in FIG. 2 for convenience ofunderstanding. FIG. 3 is a sectional view for illustrating the detailsof the optical module according to the first embodiment of the presentinvention taken along the line III-III of FIG. 2. The stem 14 has athrough hole 30 passing therethrough in a thickness direction of thestem 14. The signal lead 26 passes through the through hole 30 of thestem 14, and is electrically insulated from the stem 14. For this, thethrough hole 30 is filled with an insulating material 32, e.g., a glassmaterial, and the signal lead 26 is held inside the through hole 30 withthe insulating material 32.

The optical subassembly 10 includes a reference lead 34. The referencelead 34 is electrically connected to the stem 14 in order toelectrically connect the stem 14 to a reference potential (for example,a ground). The reference lead 34 is brazed with the stem 14.

The optical module includes a wiring substrate 36 (for example, aflexible printed board). The wiring substrate 36 is arranged on theoptical subassembly 10 (specifically, the stem 14) and is electricallyconnected thereto. The wiring substrate 36 includes the insulatingsubstrate 38. The insulating substrate 38 has a first surface 40 and asecond surface 42, and a distance between the surfaces corresponds to athickness of the insulating substrate 38. The wiring substrate 36 (firstsurface 40) is opposed to a surface of the stem 14 (a surface in whichthe through hole 30 is opened).

The wiring substrate 36 has a wiring pattern 44. The wiring pattern 44is formed on the first surface 40 and the second surface 42. The wiringpattern 44 formed on the first surface 40 overlaps the stem 14. Thewiring pattern 44 includes the plane layer 46. The plane layer 46 isformed on the first surface 40. The plane layer 46 is connected to thereference potential with the reference lead 34 that passes through partof the plane layer 46 (a connecting portion or a pad) and is solderedthereto.

The wiring pattern 44 includes signal wiring 48. The signal wiring 48 isformed on the second surface 42. The signal wiring 48 for transmittingsignals is connected to a connecting portion (or a pad) 50. The wiringpattern 44 includes the connecting portion 50. At least part of theconnecting portion 50 is formed on the first surface 40. The connectingportion 50 formed on the first surface 40 and the second surface 42passes through the insulating substrate 38, thereby forming athrough-hole via for electrical connection. The signal lead 26 is joinedto the connecting portion 50. The signal lead 26 is inserted into thethrough-hole via, and is joined to the connecting portion 50 with solder52. The solder 52 applied on the connecting portion 50 formed on thesecond surface 42 may flow through the through-hole via to reach theconnecting portion 50 formed on the first surface 40.

The wiring substrate 36 includes a protective film 54 for covering thewiring pattern 44. The protective film 54 formed on the first surface 40overlaps the stem 14. Part of the protective film 54 is opened so thatthe connecting portion 50 (through-hole via) of the wiring pattern 44may be exposed.

The optical module includes a flexible insulating layer 56. The flexibleinsulating layer 56 is formed between the optical subassembly 10 (stem14) and the wiring substrate 36. The flexible insulating layer 56 is incontact with the connecting portion 50, the signal lead 26, and thesurface of the stem 14. The signal lead 26 passes through the flexibleinsulating layer 56 to be in contact with the flexible insulating layer56. The flexible insulating layer 56 is formed of a soft dielectricmaterial, and hence the signal lead 26 can easily pierce the flexibleinsulating layer 56.

The flexible insulating layer 56 preferably has adhesiveness and a highdielectric constant (for example, a dielectric constant of 3.3). Theflexible insulating layer 56 having an adhesiveness can preventseparation of the wiring substrate 36, thereby improving solderingworkability. The wiring substrate 36 is pressed against the stem 14 sothat the flexible insulating layer 56 may absorb roughness of the stem14 and the wiring substrate 36. With this, the vicinity of the signallead 26 can be filled with the dielectric material.

According to this embodiment, the space between the connecting portion50 and the signal lead 26, the space between the connecting portion 50and the surface of the stem 14, and the space between the signal lead 26and the surface of the stem 14 are filled with the flexible insulatinglayer 56 having a dielectric constant higher than that of air. Thus,electrical connection with the stem 14 and the plane layer 46 of thewiring substrate 36 can be prompted compared to the related art in whichan air layer or a vacuumed layer is formed between the components, andhence parasitic inductance of the signal lead 26 can be reduced.Further, due to the formation of the flexible insulating layer 56, theremay be no need for a spacer between the wiring substrate 36 and the stem14, and hence a distance between the wiring substrate 36 and the stem 14can be shortened. An impedance mismatch region can therefore be reduced.

Second Embodiment

FIG. 4 is a sectional view for illustrating the details of an opticalmodule according to a second embodiment of the present invention. Inthis embodiment, an insulating material 232 is filled so that aprojecting portion 258 maybe formed that protrudes from a surface of astem 214. The projecting portion 258 is formed by the insulatingmaterial 232 creeping along a signal lead 226 by the surface tension,due to variations between lots. In such a case, a flexible insulatinglayer 256 has a recessed portion 260 in contact with the projectingportion 258. That is, the flexible insulating layer 256 absorbs theprojecting portion 258 of the insulating material 232, and hence awiring substrate 236 is mounted on the stem 214 without forming any airgap therebetween. It is therefore not necessary to consider how much theinsulating material 232 creeps up, when designing the stem 214, therebyproviding an effect of reducing the price of the stem 214. Thedescription made in the first embodiment is applied to the secondembodiment about the remaining structures.

Third Embodiment

FIG. 5 is a sectional view for illustrating the details of an opticalmodule according to a third embodiment of the present invention. In thisembodiment, a protective film 354 formed on a first surface 340 overlapsa region other than a stem 314. The protective film 354 is not formedbetween the stem 314 and a wiring substrate 336, and hence a wiringpattern 344 and a signal lead 326 can be joined to each other at aposition close to the stem 314. Impedance mismatch can therefore bereduced compared to the first embodiment, and hence excellenthigh-frequency characteristics can be expected. The description made inthe first embodiment is applied to the third embodiment about theremaining structures.

Fourth Embodiment

FIG. 6 is a sectional view for illustrating the details of an opticalmodule according to a fourth embodiment of the present invention. Inthis embodiment, a wiring pattern 444 formed on a first surface 440overlaps a region other than a stem 414. Further, a protective film 454formed on the first surface 440 overlaps a region other than the stem414.

The wiring pattern 444 includes signal wiring (not shown) formed not onthe first surface 440 but on a second surface 442, and a connectingportion 450 electrically connected to the signal wiring is formed not onthe first surface 440 but on the second surface 442. A signal lead 426is joined to the connecting portion 450 on the second surface 442 side.

The wiring pattern 444 is electrically connected to the stem 414 on thefirst surface 440 side. The stem 414 is electrically connected to aplane layer 446 of the wiring pattern 444 on a side surface of the stem414 while avoiding a surface that is opposed to a wiring substrate 436.This electrical connection is made with solder 452.

According to this embodiment, neither the signal wiring (not shown) northe connecting portion 450 is formed on the first surface 440, and hencethe stem 414 can be arranged closer to an insulating substrate 438 ofthe wiring substrate 436, thereby shortening a length of a portion whereimpedance mismatches tend to occur.

Although not illustrated, on the second surface 442, reference wiring ispreferably formed in parallel to the signal wiring (not shown) so as tobe connected to a reference potential, thereby forming a groundedcoplanar line in order to match impedances. The description made in thefirst embodiment is applied to the fourth embodiment about the remainingstructures.

Fifth Embodiment

FIG. 7 is a sectional view for illustrating the details of an opticalmodule according to a fifth embodiment of the present invention. In thisembodiment, signal wiring 548 is formed on a second surface 542 (a sideopposed to a stem 514), and a plane layer 546 is formed on a firstsurface 540 (an opposite side from the stem 514). By forming the signalwiring 548 on the second surface 542, a portion where impedancemismatches tend to occur is made shorter than in the example illustratedin FIG. 6. The signal wiring 548 is surrounded by the plane layer 546and the stem 514 to form a stripline. The description made in the firstembodiment is applied to the fifth embodiment about the remainingstructures.

EXAMPLE

Hitherto, a spacer is formed between a flexible printed board and a stemin order to prevent short-circuiting between solder and the stem. Thespacer has an opening through which a signal lead for transmittinghigh-frequency electrical signals passes, and the opening has a diameterlarger than that of the signal lead. Thus, an air layer (hereinafterreferred to as “air gap”) of about 0.1 mm is formed around a region ofthe signal lead that passes through the spacer. In addition, aninsulating material, e.g., glass for electrical insulation between thestem and the signal lead may not reach a surface of the stem due tovariations in application (for example, the state of FIG. 3), with theresult that an air gap is formed. The spacer is formed of a materialhaving a certain hardness, and hence the spacer is not deformed to fillthis air gap. The inventors of the present invention have conducted aninvestigation and found that the above-mentioned air gap region turnedinto a minute inductive region that had no negative influence ontransmission of 10 Gbit/s electrical signals, but had a negativeinfluence on transmission of high-frequency electrical signals in the 25Gbit/s class.

Then, the related-art optical module and the optical module illustratedin FIG. 5 were compared to each other by a high frequency 3Delectromagnetic field simulator (High Frequency Structure Simulator;HFSS), in terms of their reflection characteristics and transmissioncharacteristics.

FIG. 8 is a graph for showing a result of the comparison between thereflection characteristics of the related art and the third embodimentby the high frequency 3D electromagnetic field simulator (High FrequencyStructure Simulator; HFSS). FIG. 9 is a graph for showing a result ofthe comparison between the transmission characteristics of the relatedart and the third embodiment by the high frequency 3D electromagneticfield simulator (High Frequency Structure Simulator; HFSS).

In the optical module according to the related art, a reflectioncharacteristic S11 and a transmission characteristic S21 are rapidlydeteriorated at 20 GHz or more. On the other hand, in the optical moduleaccording to the third embodiment, a reflection characteristic and atransmission characteristic near 25 GHz are significantly improved.

In the optical module according to the third embodiment, the flexibleinsulating layer 56 is formed between the wiring substrate 36 and thestem 14 instead of a spacer, and hence no air gap is formed around aregion of the signal lead 26 that passes through the spacer in therelated art, that is, the flexible insulating layer 56 is formed incontact with the signal lead 26. Further, the flexible insulating layer56 is deformed when the wiring substrate 36 is mounted on the stem 14while applying stress . Thus, the flexible insulating layer 56 alsoenters an air gap region formed between the wiring substrate 36 and thestem 14 due to variations in application of the insulating material 32into the through hole 30 of the stem 14, which cannot be filled with thespacer in the related art, and hence the flexible insulating layer 56can be formed around the signal lead 26 in this region. With thiseffect, it is possible to provide an optical module having a reducedinductance and excellent high-frequency characteristics compared to theone having an air gap formed therein according to the related art. Inaddition, the flexible insulating layer 56 can be thinner than thespacer, and hence an effect of further reducing an inductance comparedto the case using the spacer is obtained. Further, as described above,the air gap formed due to variations in application of the insulatingmaterial 32 into the through hole 30 of the stem 14 can also be filled,and hence an inexpensive stem can be used, thereby being capable ofproviding an inexpensive optical module.

It is desired that the air gap formed around the signal lead 26 beideally completely filled with the flexible insulating layer 56. Thesignal lead 26 may not be completely surrounded in some cases due tovariations in manufacturing. Even in such a case, however, the flexibleinsulating layer 56 increases effective dielectric constant around thesignal lead 26, and hence an inductance reduction effect is obtainedcompared to the case in which a large air gap is formed as in therelated art. Thus, the flexible insulating layer 56 does not necessarilycompletely surround the signal lead 26.

While there have been described what are at present considered to becertain embodiments of the invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaims coverall such modifications as fall within the true spirit andscope of the invention. For example, in the embodiments described above,the flexible insulating layer 56 is exemplified as a soft layer that thesignal lead 26 can pierce, but the flexible insulating layer 56 is notlimited thereto. More specifically, the flexible insulating layer 56maybe a layer that has flexibility in a manufacturing step of connectingthe stem 14 and the wiring substrate 36 to each other, and is cured tolose its flexibility thereafter through application of a certain energy.As an example, a UV curing agent is given. A UV curing agent is amaterial that is a liquid having viscosity in an initial state, and issolidified through irradiation of UV rays. With the use of, for example,a UV curing agent, an effect equivalent to that of the invention of thesubject application is obtained. A flexible insulating layer used hereinrepresents an insulating layer having flexibility in a manufacturingstep of connecting a stem and a wiring substrate to each other.

What is claimed is:
 1. An optical module, comprising: an opticalsubassembly configured to convert at least one of an optical signal oran electrical signal into another one of the optical signal and theelectrical signal; a wiring substrate that has a wiring patterncomprising a connecting portion and is arranged on the opticalsubassembly so as to be electrically connected thereto; and a flexibleinsulating layer formed between the optical subassembly and the wiringsubstrate, the optical subassembly comprising: a conductive stem thathas a surface opposed to the wiring substrate, the conductive stem beingshaped so that the surface has a through hole opened therein and beingconnected to a reference potential; and a signal lead for transmitting asignal, the signal lead passing through the through hole while beingelectrically insulated from the conductive stem, the signal lead passingthrough the flexible insulating layer to be joined to the connectingportion, the flexible insulating layer being in contact with theconnecting portion, the signal lead, and the surface of the conductivestem.
 2. The optical module according to claim 1, wherein the wiringsubstrate comprises an insulating substrate having a thicknesscorresponding to a distance between a first surface and a second surfaceof the insulating substrate, wherein the first surface is opposed to thesurface of the conductive stem, and wherein at least part of theconnecting portion is formed on the first surface.
 3. The optical moduleaccording to claim 2, wherein the connecting portion is formed on thefirst surface and the second surface so as to pass through theinsulating substrate, to thereby form a through-hole via for electricalconnection, and wherein the signal lead is inserted into thethrough-hole via.
 4. The optical module according to claim 2, whereinthe wiring pattern comprises signal wiring connected to the connectingportion, for transmitting the signal, and a plane layer for connectionwith the reference potential.
 5. The optical module according to claim4, wherein the signal wiring is formed on the second surface, andwherein the plane layer is formed on the first surface.
 6. The opticalmodule according to claim 4, wherein the signal wiring is formed on thefirst surface, and wherein the plane layer is formed on the secondsurface.
 7. The optical module according to claim 2, wherein the wiringsubstrate comprises a protective film for covering the wiring pattern,and wherein the wiring pattern is formed on the first surface and thesecond surface.
 8. The optical module according to claim 7, wherein thewiring pattern formed on the first surface overlaps the conductive stem.9. The optical module according to claim 7, wherein the wiring patternformed on the first surface overlaps a region other than the conductivestem.
 10. The optical module according to claim 7, wherein theprotective film formed on the first surface overlaps the conductivestem.
 11. The optical module according to claim 7, wherein theprotective film formed on the first surface overlaps a region other thanthe conductive stem.
 12. The optical module according to claim 1,wherein the optical subassembly further comprises a reference leadelectrically connected to the conductive stem in order to electricallyconnect the conductive stem to the reference potential.
 13. The opticalmodule according to claim 1, wherein the wiring pattern is electricallyconnected to the conductive stem on the first surface, and wherein theconductive stem is electrically connected to the wiring pattern on aside surface of the conductive stem while avoiding the surface.
 14. Theoptical module according to claim 1, wherein the through hole of theconductive stem and the signal lead are electrically insulated from eachother with an insulating material, wherein the insulating material isfilled so that a projecting portion is formed that protrudes from thesurface of the conductive stem, and wherein the flexible insulatinglayer has a recessed portion in contact with the projecting portion.